IMPY (2,3-dihydro-1H pyrazolo (2,3-a) imidazole, (NSC-51143) is a new anti-tumor drug currently being evaluated in Phase I-II clinical trials. Although it inhibits the activity of ribonucleotide reductase, it is active in vivo against L1210 leukemic cells resistant to other inhibitors of the enzyme such as TSC and Guanozole. It also lacks the schedule dependency usually seen with cell-cycle specific agents. The possibility of either an active metobolite or another mechanism of action exists. We have found the drug to be extensively metabolized in the dog and in man and have synthesized four metabolites. Preliminary studies suggest that a beta-ribosyl derivative may have activity. IMPY causes significant hemolysis in patients. Pursuing this effect, we have shown oxygen dependent hexose monophosphate shunt stimulation with hydrogen peroxide production in red blood cells after incubation with IMPY. This raises the possibility of oxygen radical formation. We have also demonstrated depletion of ATP in red cells incubated with IMPY. We proposed to systematically study IMPY and its metabolites in regards to generation of oxygen radicals, anti-tumor effect in L1210 leukemia, inhibition of DNA synthesis and depletion of ATP stores.